Emeralds are thought to be unlucky because green is associated with bad luck in many cultures. I know my Dad always told me never to buy a green car as it was unlucky (but I thought that was just his "personal" superstition). But in ancient Egypt they were used as a healing some and believe it has a re-balancing effect on the psyche, even promoting clairvoyancy.
I'm off to ask my psychic if it's true!!!!